This proposal aims to develop a mechanistic-based genotoxicity assay that is rapid and inexpensive and measures three genotoxicity endpoints. The assay involves three genetically engineered strains of yeast, one for each endpoint. The strains are designed to assess the effect of suspected toxins on a particular yeast mechanism, thus the assay is mechanistic-based. The assay can be used in place of standard mutagenicity assays and provides mode-of-action data that could prove valuable in human risk assessment of carcinogenicity. PROPOSED COMMERCIAL APPLICATION The assay proposed herein is rapid and inexpensive and has advantages over commercially available assays with similar properties. The assay will be useful to industries, the EPA, NIH, DOD and other government agencies where toxic chemicals are employed.